Serotonin, i.e. 5-hydroxytryptamine (5-HT), is a tissue hormone released from thrombocytes and constricting blood vessels, and also a neurotransmitter in the brain. Serotonin depletion may manifest itself as melancholy, apathy, depression, insomnia, change of appetite and chronic fatigue. In addition to its central nervous system activity, serotonin regulates bowel motility and sensing of pain. Serotonin depletion in the body has also been substantially associated with different disturbances in the intestines, such as IBS (irritable bowel syndrome) and “leaky gut”.
A low serotonin level thus presents a health risk. Serotonin depletion can be treated by various drugs, such as selective serotonin reuptake inhibitors (SSRI drugs) or monoamine oxidase inhibitors (MAO inhibitors). Like drugs in general, also those raising the serotonin level may have more or less harmful side effects.
The present invention now offers an alternative for raising the serotonin level. The invention also provides means for determining a health risk associated with a low serotonin level, for reducing the risk and for selecting the right form of therapy. The invention makes use of the gut microbiota of the target.
The composition of gut microbiota has been demonstrated to have numerous effects on the wellbeing and health of the host (Hooper L. V., Gordon J. Science 2001; 292:1115-8; and Bäckhed, F et al. 2005. Science 307, 1915-1920). Use probiotic bacteria has been suggested for promoting development of early bifidogenic gut microbiota in order to reduce the risk of overweight or obesity in small children later in life (EP 1 974 734). Rats have been fed with bifidobacteria, and the rats that received bifidobacteria were found to have more tryptophan and kynurenic acid in plasma and less inflammation factors (IFN-γ, TNF-α and IL-6) than the control animals. Tryptophan is a precursor of serotonin (Desbonnet L et al. J Psychiatr Res. 2008; 43:164-174). The authors speculate that bifidobacteria may have anti-depressant effects. They emphasize, however, that the results are preliminary and must be interpreted with caution. Although bifidobacteria raised peripheral tryptophan level, they did not raise the serotonin level in the central nervous system. Neither did bifidobacteria have any effect on the behaviour of rats in a forced swim test, which is for testing antidepressants. Further, it is to be kept in mind that results obtained in animal testing are not directly applicable to humans. Another point worth mentioning is that leptin acts differently in rats than in humans. It is also to be remembered that so far the understanding of the functioning of the brain-gut connection and gut microbes possibly regulating it is fairly inadequate. The present invention enhances this understanding.